Field of the Invention
This invention is directed to an improved process for electroplating a conductive metal to the surface of a nonconductive material. In particular, this invention relates to a process for preparing the through hole walls of a printed wiring board (PWB) for electroplating. Further, this invention relates to a new liquid conductive graphite dispersion for preparing nonconductive material for electroplating. Still further, the present invention relates to the resulting nonconductive material prepared by the above-noted process.
Conventional electroless processes have several commercial disadvantages. They require a relatively long process time. The multiple treatment baths have complex chemistry which may require constant monitoring and individual ingredients which may require separate replenishment. The conventionally used palladium/tin activator also may require expensive waste treatment. Furthermore, these electroless process baths may be very sensitive to contamination. Finally, the multiplicity of rinse baths may require large amounts of water.
Prior to the electroless method of plating through holes, graphite was employed to prepare the walls of the through holes for plating. For example, U.S. Pat. No. 3,099,608, which issued to Radovsky et al. on Jul. 30, 1963, teaches a process for preparing the through hole walls of printed circuit boards (also called printed wiring boards or PWB's) for electroplating by initially depositing in said through holes a thin electrically nonconductive film of palladium metal in at least a semi-colloidal form. The patent discloses that graphite had been used previously as a conductive layer for electroplating thereon. See column 1, lines 63-70 and column 4, line 72 to column 5, line 11. These patentees noted several deficiencies with that graphite process including lack of control of the graphite application, poor deposit of the resultant electroplated metal, nonuniform through hole diameters, and low electrical resistance of the graphite.
U.S. Pat. No. 3,163,588, which issued to Shortt et al. on Dec. 29, 1964, also mentions that graphite or its equivalents may be employed to render through hole walls of electric circuit boards conductive for later electroplating metals thereon. See column 3, line 45 to column 4, line 2.
U.S. Pat. No. 4,581,301, which issued to Michaelson on Apr. 8, 1986, teaches the application of a seed layer of conductive particles, such as "carbon", on the walls of through holes before electrolytically plating copper over the seed layer. This reference does not explicitly teach the use of a continuous layer of carbon black dispersion in the seed layer, and does not recognize the advantage of using very small particles of carbon black such as presently claimed. See column 7, lines 63-66 which refer to particles passing through a 400 mesh screen. A 400 mesh screen is equivalent to about 35 microns.
Separately, graphite has been employed in numerous processes for preparing a nonconducting material for a metal coating or plating For example, U.S. Pat. No. 409,096, which issued to Alois Blank on Aug. 13, 1889, teaches a process for applying copper to asbestos roofing material which comprises first applying powdered plumbago (graphite) in a volatile liquid such as varnish to the surface of the asbestos, then evaporating the volatile liquid to coat the asbestos fibers with fine particles of plumbago. The plumbago coated asbestos sheets are then immersed in a copper electroplating solution and electric current is applied to the coated asbestos sheet to form a thin film of copper thereon. The copper coated sheet is then immersed in a bath of molten metal such as tin, lead, or zinc, and is then removed from the molten bath to effect solidification of the molten metal. The resulting metal coated asbestos sheet is described as being relatively flexible, a nonconductor of heat and substantially fireproof.
U.S. Pat. No. 1,037,469, which issued to Goldberg on Sep. 3, 1912, and U.S. Pat. No. 1,352,331, which issued to Unno on Sep. 7, 1920, disclose processes for electroplating nonconducting materials by first coating the nonconducting material with wax, then coating the wax with a slurry of finely divided particles of graphite or other metal, followed by electroplating of the dust-coated surface with copper or other metal. Neither of these processes are particularly suitable for use in coating the hole walls of circuit boards because the holes are normally extremely narrow in diameter and immersing in wax would tend to plug the hole and prevent coating the hole walls with an electroplating material.
U.S. Pat. No. 2,243,429, which issued to Laux on May 27, 1941, discloses a process for electroplating a nonconductive surface by "graphiting" a thin layer onto the nonconducting surface followed by applying a copper layer electrolytically and "finally a further electrolytic deposit of another metal" is placed thereon.
Separately, carbon black formulations have been employed as conductive coatings for nonconductive materials. For example, U.S. Pat. No. 4,035,265, which issued to Saunders on Jul. 12, 1977, discloses conductive paint compositions containing both graphite and carbon black along with air-hardenable binder. These paints are suitable for application to the walls of a building for use as a heating element.
U.S. Pat. No. 4,090,984, which issued to Lin et al. on May 23, 1978, teaches a semiconductive coating for glass fibers comprising (a) a polyacrylate emulsion; (b) electrically conductive carbon black dispersion and (c) a thixotropic gelling agent. The conductive carbon black dispersions employed are those comprising electrically conductive carbon black dispersed, from about 3 to about 4% by weight, in a suitable dispersing agent.
U.S. Pat. No. 4,239,794, which issued to Allard on Dec. 16, 1980, teaches dispersing a conductive carbon black in a latex binder with a selected dispersing agent, then impregnating this carbon black dispersion into a nonwoven fibrous web followed by drying any residual water, leaving a thin coating of carbon black dispersed on the surfaces of said fibers.
U.S. Pat. No. 4,619,714, which issued Oct. 28, 1986, and its divisional, U.S. Pat. Nos. 4,684,560, which issued Aug. 4, 1987 and 4,724,005, which issued Feb. 9, 1988, describe a process for electroplating a conductive metal to the surface of a nonconductive material, particularly a process of electroplating the through holes of a printed wiring board. This process is a significant improvement over the known electroless techniques. By this process, a liquid dispersion of carbon black particles is first applied to the nonconductive material, such as the nonconductive portions of through holes on a printed wire board; then the liquid dispersion medium is separated (i.e., evaporated) from the carbon black particles, thereby depositing a substantially continuous layer of carbon black particles on the nonconductive surface; and next a substantially continuous metal layer is electroplated over the deposited carbon black layer. This process of U.S. Pat. No. 4,619,714 has several advantages over the known electroless techniques including the elimination of the preactivator, the Pd/Sn activator, and the accelerator; less possibility of pollution problems; better bath stability; and fewer possible side reactions.
In Comparisons 1 and 2, in columns 19 and 20 of U.S. Pat. No. 4,619,714, two formulations containing graphite were prepared and circuit boards were treated with the formulations. These graphite formulations employed relatively large size graphite particles. The mean particle size of the solids in these Comparisons was found to be 3.1 microns. The circuit board was then plated. The patent states in column 20, lines 16-20, that in all, both graphite formulations were far inferior for copper electroplating preparation as compared to formulations containing carbon black. The reason why these Comparisons failed may be attributed in part to the relative large size of the graphite particles in that dispersion.
Improvements and modifications of the electroplating process of U.S. Pat. Nos. 4,619,741, 4,684,560, and 4,724,005 are described in the following patents:
U.S. Pat. No. 4,622,107, which issued Nov. 11, 1986, describes the use of a gas-forming compound (e.g., sodium carbonate) to remove loosely or easily removable carbon black particles in the through holes.
U.S. Pat. No. 4,622,108, which issued Nov. 11, 1986, describes the contacting of an alkaline hydroxide preconditioning solution to the through-hole walls before application of the carbon black dispersion so that the carbon black dispersion will have better adhesion to the walls.
U.S. Pat. No. 4,631,117, which issued Dec. 23, 1986, describes the use of the carbon black dispersion described in U.S. Pat. No. 4,619,714 as a preactivator for electroless plating of the through holes.
U.S. Pat. No. 4,718,993, which issued Jan. 12, 1988, describes the use of an aqueous alkaline silicate solution to contact a printed wiring board prior to contacting with the carbon black dispersion.
U.S. Pat. No. 4,874,477, which issued Oct. 7, 1989, describes contacting a printed wire board with a particular aqueous polyelectrolyte homopolymer conditioner followed by contacting the printed wiring board with the carbon black dispersion.
U.S. Pat. No. 4,897,164, which issued Jan. 30, 1990, describes contacting a printed wiring board with an aqueous solution of an alkali metal borate after it has been contacted with the carbon black dispersion and prior to microetching.
U.S. Pat. No. 4,964,959, which issued Oct. 23, 1990, describes the addition of a conductive polymer or combinations thereof to the carbon back dispersion.
U.S. Pat. No. 4,994,153, which issued Feb. 19, 1991, describes a process for treating the tooling holes or slots which have been coated with a carbon black dispersion in a nonconductive material which comprises removing said carbon black with an aqueous solution containing: (a) an alkanolamine; (b) an anionic surfactant which is the neutralized addition product of maleic and/or fumaric acid and a poly(oxylated) alcohol; (c) a nonionic surfactant which is an aliphatic mono and/or diphosphate ester; and (d) an alkali or alkaline earth metal hydroxide.
U.S. Pat. No. 5,015,339, which issued on May 14, 1991, describes an electroplating pretreatment wherein nonconductive material is first contacted with an alkaline permanganate solution, then a neutralizer/conditioner solution and then a carbon black dispersion.
All of the above-noted U.S. patents are incorporated herein by reference in their entireties.
While the above patents describe an effective means for electroplating a metal to the surface of a nonconductive material, particularly the through holes of a printed wiring board (PWB), there is still a desire to improve the overall quality of that electroplating process.
One of the limiting factors in the above-noted carbon black dispersion preplating process, described generally in U.S. Pat. Nos. 4,619,714; 4,684,560; and 4,724,005 and their above-noted improvement patents, is the plating speed of copper over the carbon black deposit, or in other words, the time needed to achieve complete coverage of the through holes by electrodeposited copper. Currently, the copper plating speed in PWB through holes is restricted by the relatively high resistance of the carbon black films. It has now been found that the deposition of a conductive graphite layer on top of the carbon black layer enhances the overall conductivity of the carbon black coating and, therefore, yields faster plating rates. Some of the patents mentioned above have described the use of graphite. However, such graphite films are usually plagued by adhesion problems. In the process of the present invention, fine particle-size conductive graphite is deposited on top of a carbon black deposit. The carbon black deposit serves as an adhesion promoter for the graphite film. The conductivity and particle size of graphite particles are critical parameters of the present invention.